Catalog of Earth Satellite Orbits

By Holli RiebeekDesign by Robert SimmonSeptember 4, 2009

Just as different seats in a theater provide different perspectives on a performance, different Earth orbits give satellites varying perspectives, each valuable for different reasons. Some seem to hover over a single spot, providing a constant view of one face of the Earth, while others circle the planet, zipping over many different places in a day.

There are essentially three types of Earth orbits: high Earth orbit, medium Earth orbit, and low Earth orbit. Many weather and some communications satellites tend to have a high Earth orbit, farthest away from the surface. Satellites that orbit in a medium (mid) Earth orbit include navigation and specialty satellites, designed to monitor a particular region. Most scientific satellites, including NASA’s Earth Observing System fleet, have a low Earth orbit.

One way of classifying orbits is by altitude. Low Earth orbit starts just above the top of the atmosphere, while high Earth orbit begins about one tenth of the way to the moon. (NASA illustration by Robert Simmon)

The height of the orbit, or distance between the satellite and Earth’s surface, determines how quickly the satellite moves around the Earth. An Earth-orbiting satellite’s motion is mostly controlled by Earth’s gravity. As satellites get closer to Earth, the pull of gravity gets stronger, and the satellite moves more quickly. NASA’s Aqua satellite, for example, requires about 99 minutes to orbit the Earth at about 705 kilometers up, while a weather satellite about 36,000 kilometers from Earth’s surface takes 23 hours, 56 minutes, and 4 seconds to complete an orbit. At 384,403 kilometers from the center of the Earth, the Moon completes a single orbit in 28 days.

The higher a satellite’s orbit, the slower it moves. Certain orbital altitudes have special properties, like a geosynchronous orbit, in which a satellite travels around the Earth exactly once each day. The length of each red arrow in this diagram represents the distance traveled by a satellite in an hour. View animation. (NASA illustration by Robert Simmon.)

Changing a satellite’s height will also change its orbital speed. This introduces a strange paradox. If a satellite operator wants to increase the satellite’s orbital speed, he can’t simply fire the thrusters to accelerate the satellite. Doing so would boost the orbit (increase the altitude), which would slow the orbital speed. Instead, he must fire the thrusters in a direction opposite to the satellite’s forward motion, an action that on the ground would slow a moving vehicle. This change will push the satellite into a lower orbit, which will increase its forward velocity.

In addition to height, eccentricity and inclination also shape a satellite’s orbit. Eccentricity refers to the shape of the orbit. A satellite with a low eccentricity orbit moves in a near circle around the Earth. An eccentric orbit is elliptical, with the satellite’s distance from Earth changing depending on where it is in its orbit.

The eccentricity (e)
of an orbit indicates the deviation of the orbit from a perfect circle. A circular orbit has an eccentricity of 0, while a highly eccentric orbit is closer to (but always less than) 1. A satellite in an eccentric orbit moves around one of the ellipse’s focal points, not the center. (NASA illustration by Robert Simmon.)

Inclination is the angle of the orbit in relation to Earth’s equator. A satellite that orbits directly above the equator has zero inclination. If a satellite orbits from the north pole (geographic, not magnetic) to the south pole, its inclination is 90 degrees.

Orbital inclination is the angle between the plane of an orbit and the equator. An orbital inclination of 0° is directly above the equator, 90° crosses right above the pole, and 180° orbits above the equator in the opposite direction of Earth’s spin. (NASA illustration by Robert Simmon.)

Together, the satellite’s height, eccentricity, and inclination determine the satellite’s path and what view it will have of Earth.